1. Field of the Invention
The present invention relates to a methyl methacrylate resin composition which exhibits little drawdown in heat molding. This resin composition is suitable for the production of molded articles by extrusion molding, blow molding and expansion molding, especially for the production of large molded articles.
2. Description of Related Art
Methyl methacrylate polymers have sufficient rigidity, excellent transparency and excellent weather resistance. Thus the polymers have been widely used for molded articles by injection molding, such as lamp covers and meter covers of automobiles, spectacle lenses and light guide members, and also for extruded boards by extrusion molding, such as signboards and nameplates.
On the other hand, conventional methyl methacrylate polymers can afford only small molded articles because the tension of the melt oriented resin composition is small in blow molding, and therefore much drawdown is caused. Furthermore, it can be molded by foaming only under limited conditions in temperature, molding pressure and the like. Thus resin compositions with both high flowability and little drawdown have been desired.
With regard to improving processing properties in molding, Japanese Unexamined Patent Publication (Kokai) No. 5-140411 has disclosed a method in which polytetrafluoroethylene is added.
Furthermore, U.S. Pat. No. 5,726,268 has disclosed that methyl methacrylate polymer with a branched structure have high tension in melt orientation with retaining melt flowability.
However, in the method wherein polytetrafluoroethylene is added, the drawdown is improved, but sufficient tension in melt orientation necessary for blow molding in large scale can not be achieved. Therefore, the effect of that method is insufficient. In addition, the transparency, which is one of characteristics of acrylic resin, is also lost because polytetrafluoroethylene has a refractive index different from that of methyl methacrylate polymer.
Although the use of the methyl methacrylate polymer with a branched structure can afford acrylic resin having both higher flowability and higher tension in melt orientation compared with those at the time when a conventional linear methyl methacrylate polymers, such acrylic resin does not have drawdown low enough to make the resin applicable also to blow molding in large scale in which a parison having a length more than 40 cm is used. Thus resin which can exhibit still lower drawdown in melt orientation for producing large scale articles has been demanded.